Growing nerve terminals in the brain have been reconstructed from serial sections of freeze-substituted brain. These new preparative methods have revealed an internal system of membranes which are thought to be the source of the new membrane added to the surface of the growth cone during its growth. These membranes are highly labile and are destroyed by conventional fixatives. Our new evidence, based on uptake of structural tracers, shows that these membranes participate in recycling of the membranes needed for extension of the growth cone and also function as intracellular compartments where the plasmalemmal surface of the growth cone can be modified during neuronal development. Other evidence from video analysis of axonal outgrowth of cultured neurons indicates that axonal outgrowth is derived from the proximal growth cone and the growth cone plasmalemma is, in part, derived from the anterogradely transported vesicles and vacuoles which move next to and then fuse with it. We have also performed shallow-etch structural analysis of adult synapses in cerebellar cortex and discovered that a meshwork of filamentous proteins surrounds synaptic vesicles near the axon. Molecular shadowing of pure synapsin I tentatively identifies this protein as a major component of the presynaptic meshwork.